This invention relates to methods of operating a fluidized bed system combining a dense fluidized bed and an entrained fluidized bed. More particularly it relates to the operation of a system wherein the beds contain two different particle components, at least one component having long-term physical and chemical stability in the system and being retained in the dense fluidized bed while the other is entrained and recirculated therethrough.
Fluidized beds operated according to the methods disclosed herein can be utilized in a multitude of processes, from simple heat-exchange reactions through ordinary catalytic reactions to complex chemical reactions. However, our methods will be described and illustrated in connection with the operation of a fluidized-bed combustor for burning high-sulfur coal, to provide heat for a boiler while reducing noxious emissions to almost any reasonably desired extent.
Good general discussions of the characteristics of fluidized beds for deriving energy products such as heat and fuel gas from coal can be found in U.S. Pat. Nos. 2,638,684, 2,665,200, and 3,840,353 and references cited therein. One general type of coal gasification plant which has been proposed employs a so-called moving burden of essentially inert material to carry heat from a heating vessel or combustion vessel to a gasification vessel. According to U.S. Pat. No. 2,654,665, the inert material is ash, which is heated by burning carbon particles, that are mixed with the ash, in a combustion vessel or in its entrance conduit. The combustion vessel contains a fluidized bed operated at a temperature of 1950.degree. F. It is known that coal ash agglomerates at this temperature to form increasingly larger particles, as explained in U.S. Pat. No. 3,840,353 supra. The large particles of ash remain in the fluidized bed in the combustion vessel until they are discharged through a draw-off conduit, whereas the finer ash particles are entrained and carried out of the combustion chamber. They are separated from the fluidizing gas, passed through the gasification vessel to supply heat for the endothermic reaction, and then returned to the combustion vessel to be reheated.
According to another proposal for a gasification plant of the same general type, disclosed in U.S. Pat. No. 2,741,549, a dense fluidized bed in the combustion vessel contains coarse sand. The velocity of the gas is chosen so that coarse solids remain in the bottom portion of the vessel, while fine solids are entrained in the bottom portion and move farther up to form a thin dense fluidized layer of fine solids on top of the bottom dense fluidized bed. The formation of this layer is effected by widening the vessel to reduce the gas velocity and/or by the use of a screen or tower packing. The fine solids in the layer are then drawn off through a conduit and fed into the gasification vessel to supply heat thereto.
According to still another proposal for a gasification plant of the same general type, disclosed in U.S. Pat. No. 2,979,390, only fully entrained fluidized beds are contained in both the heating vessel and the gasification vessel, and an additional supply of heat for the gasification vessel is brought in by a flow of a so-called thermophore. The thermophore comprises particles of a material which is readily separable from coal and ash, having a high fusion temperature, high specific heat and high specific gravity. The thermophore is heated in a separate vessel, to a temperature higher than the fusion temperature of ash, passed through the entrained fluidized bed in the gasification vessel, separated from the entrained coal and returned through its own separate heating vessel. This arrangement can be contrasted with that of U.S. Pat. No. 2,638,684 supra, wherein a dense fluidized bed of inert materials remains in the reactor, whereas the coal particles are entrained, without, however, being recirculated through the dense fluidized bed.
There have also been a number of recent proposals to provide commercial fluidized bed boiler systems. According to some of these proposals, ground coal is fed into a bed of limestone particles that are fluidized with the air which supports combustion. The limestone acts as a sorbent for the sulfur in the coal, since the sulfur combines with oxygen from the air to form sulfur dioxide, and the sulfur dioxide reacts with the limestone to form calcium sulfate. The calcium sulfate can be disposed of with the ash from the burned coal, or if desired the sulfur can be recovered and the limestone regenerated.
In comparison with conventional pulverized coal-fired boilers, fluidized bed systems offer the advantages of reduced boiler size, increased efficiency and flexibility and the ability to burn highly caking coal. Moreover, because they operate at a lower temperature, i.e., a temperature around 1550.degree. F which optimizes the efficiency of the CaSo.sub.4 - forming reaction, there is reduced NO.sub.x emission as well as reduced SO.sub.2 emission, and reduced steam tube corrosion and fouling.
Fluidized bed combustors operated in accordance with the methods of the present invention retain these advantages and additionally provide other advantages, including still further reductions in boiler size and heat transfer surface requirements. Because of these reductions, there can be a significant decrease in the capital cost of industrial boiler systems, savings in construction materials, labor, transportation costs, and avoidance of the necessity for field erections of many boilers, which can be very compact, shop-fabricated and railroad or truck-transportable.
The generation of a high output of heat in a compact unit necessitates a high throughput of coal, which in turn requires a high volume of air per unit time to burn the coal, that is, a high velocity of air passing through the fluidized bed. This air and the resulting combustion gases also serve the purpose of fluidizing the bed, and its velocity is commonly indicated by the term "superficial velocity." The superficial velocity is calculated by dividing the volume of gas per unit time, passing through the combustor, by the cross-sectional area of the combustor normal to the principal direction of the air flow. Hence the superficial velocity is the velocity the gas would have if the combustor were empty of the fluidized bed particles which it normally contains.
Previous fluidized-bed boiler combustors have generally been limited to operation at superficial velocities no greater than around 10-14 feet per second, and usually considerably less, because at high velocities substantial quantities of carbonaceous particles and limestone particles are elutriated or blown out of the combustor long before they can be completely burned or sulfated. On the other hand, a combustor has been very satisfactorily operated in accordance with this invention at a superficial velocity greater than 30 feet per second, and there is no apparent reason that this can not be increased to perhaps 100 feet per second or so, with suitable adjustment of other operating parameters.
In accordance with one typical procedure, instead of the conventional limestone bed in the combustor we emply two solid particle components, at least one of the components essentially comprising a material having long-term physical and chemical stability in the combustor system. One specific material we have successfully used to form one or both components is a hematite ore, containing about 93% of an oxide of iron, Fe.sub.2 O.sub.3, and supplied under the trademark "Speculite" by C.E. Minerals, Inc. of King of Prussia, Pennsylvania. The first component may consist of "fine" particles of this ore in the range of about -16 + 140 U.S. mesh; that is, the particles will pass through a 16 mesh screen but not through a 140 mesh screen. A suitable alternate first component particle may comprise limestone particles in the range of about -20 + 40 U.S. mesh. In both cases the second component consists of "coarse" Speculite particles in the range of about -12 + 16 U.S. mesh. The bed system containing these fine and coarse particles is fluidized with gas at a superficial velocity of about 30 feet per second.
At this velocity, the fine hematite or limestone particles are carried along with the current of air, forming an entrained fluidized bed which is highly expanded to fill substantially the entire space region encompassed by the main combustor chamber and its exit conduit. The coarse hematite particles are too massive to be entrained, but form a dense fluidized bed which is retained in a more limited space region at the bottom of the main combustor chamber. The fine particles are carried out of the main combustor chamber and into a separator, such as a cyclone separator, whereby the fine particles are removed from the entraining gas stream and fed into a recirculation path. The recirculation path carries the fine particles back through the dense fluidized bed. Hence there is a continuous, recirculating flow of entrained fine particles through the agitated, fluidized mass of coarse particles.
Ground coal is fed into the dense fluidized bed containing the commingled coarse and fine particles and burned to produce heat. The heat is removed by passing a heat transfer medium, such as water contained in boiler tubes, through the region of the entrained fluidized bed as well as the region of the dense fluidized bed. Pulverized limestone having a typical particle size of -325 U.S. mesh is fed in with the coal. The temperature in the combustor is maintained at about 1550.degree. F to promote the efficacy of the limestone as a sulfur sorbent.
The numerous advantages of a fluidized bed boiler operated in accordance with the present invention can be explained by an examination of its characteristics. The recirculating fine bed component particles which interpenetrate the dense fluidized bed appear to provide highly uniform fluidization and minimize "slugging". The agitated motion of the commingled particles results in thorough mixing and intimate contact between the gaseous and solid reactants introduced into the dense fluidized bed. Even though the superficial velocity is unprecedentedly high, the commingled bed particles substantially retard the movement of the coal and pulverized limestone particles in the principal direction of air flow. Hence the coal particles are retained in the dense fluidized bed for a sufficient length of time to allow the major portion of the coal particles to be completely burned before they are carried out of the dense bed region. Similarly the residence time of the pulverized limestone particles is made sufficient to insure its effectiveness as a sorbent.
It is apparent that the limestone surface area available to react with the sulfur dioxide, for a given weight of limestone, can be vastly increased by finer grinding, as is done in the case of agricultural limestone. However, it has not previously been possible to utilize the finely-ground limestone in high-velocity fluidized bed combustors because the small particles would simply be blown out of the combustor before they could absorb any substantial quantity of sulfur. Hence coarse limestone has been used according to most of the prior proposals, depending on attrition to keep new surfaces continuously exposed. However, limestone with satisfactory attrition characteristics is not readily available in all parts of the world. The present methods make it possible in all probability to use limestone from anywhere in the world, without concern for its attrition characteristics in a fluidized bed.
The increased residence time for the fine coal particles or the pulverized limestone particles in the dense fluidized bed portion of the illustrative combustor, operated in accordance with the methods of this invention at high superficial velocities, probably occurs because the coarse bed component particles limit the mean free path of the fine bed component particles, and both of these bed component particles limit the mean free path of the coal and limestone particles in the dense bed region.
The use of finely ground, pulverized limestone as a sulfur sorbent, in the manner made possible by the methods of the present invention, somewhat reduces the consumption of limestone and thereby reduces the quantity of the resulting sulfated limestone to be disposed of. The efficiency of limestone utilization is further improved by the grinding effect of the hematite, which continually provides fresh limestone surfaces for sulfur dioxide absorption.
The high throughput per unit volume, and high heat release rate of a fluidized bed boiler operated according to the methods of the present invention are achieved in part by the high heat transfer rate throughout the entire volume of the main combustion chamber, including what is normally the freeboard region above the dense fluidized bed. This freeboard region as well as the dense fluidized bed region may contain boiler tubes which receive a high input of heat transmitted by the fine bed component particles permeating the entire boiler tube space because of their entrainment in the gas stream.
A fluidized bed boiler operated according to this invention nevertheless can provide high turndown ratios, and is therefore controllable to suit widely varying load requirements. By reducing the coal feed rate and the concommitant air flow, the boiler can be turned down until it is operating at low heat output and as a conventional fluidized bed with no entrainment of the fine bed particles.